1. Field of the Invention
The present invention relates to a liquid crystal display device, which is constituted by sealing liquid crystals between two substrates, and to methods of manufacturing the same.
2. Description of the Prior Art
Liquid crystal display devices have a feature that they are thin and light and that they have low power consumption; and, recently, they have come to be widely used for displays for various kinds of electronic devices. A usual liquid crystal display device has a structure in which liquid crystals are sealed between two substrates that are disposed to face each other. On one substrate, thin film transistors (TFTs), picture element electrodes, and the like are formed, while color filters, a common electrode, and the like are formed on the other substrate. Hereinafter, a substrate on which TFTs, picture element electrodes, and the like are formed is referred to as a TFT substrate; and a substrate, which is disposed to face the TFT substrate, is referred to as an opposing substrate. A structure formed by sealing liquid crystals between the TFT substrate and the opposing substrate is referred to as a liquid crystal panel.
FIG. 1 is a plan view showing an example of a liquid crystal display device, and FIG. 2 is a schematic sectional view taken along line I-I of FIG. 1. Although FIG. 1 shows a region for one picture element, in practice, a large number of picture elements are arranged in matrix in the horizontal direction (in the direction of X-axis) and in the vertical direction (in the direction of Y-axis).
As shown in FIG. 2, a liquid crystal panel 1 includes a TFT substrate 10, an opposing substrate 20, and a liquid crystal layer 30 formed of liquid crystals which are sealed between these substrates 10 and 20. Here, the liquid crystal layer 30 is constituted by liquid crystals with negative dielectric anisotropy; and when a voltage is not applied, liquid crystal molecules are aligned in the direction perpendicular to the substrate surfaces.
A first polarizing plate 31a is disposed on the back side (on the lower side in FIG. 2) of the liquid crystal panel 1, and a second polarizing plate 31b is disposed on the front side (on the observer's side/on the upper side in FIG. 2) thereof. In addition, a backlight (not shown) is disposed on the back side of the liquid crystal panel 1. Here, the first polarizing plate 31a and the second polarizing plate 31b are disposed in such a way that the absorption axes of the polarizing plates are perpendicular to each other. In this case, display is a black display when a voltage is not applied.
As shown in FIG. 1, on a glass substrate 10a which becomes a base for the TFT substrate 10, a plurality of gate bus lines 11 extending in the horizontal direction and a plurality of data bus lines 15 extending in the vertical direction are formed. Rectangular regions, which are divided by the gate bus lines 11 and the data bus lines 15, are picture elements regions, respectively. The gate bus lines 11 are covered with a first insulating film 12, and the data bus lines 15 are formed on the first insulating film 12.
On the TFT substrate 10, a TFT 16 and a picture element electrode 18 are formed for every picture element region. In this example, for the TFT 16, a part of the gate bus line 11 is a gate electrode, and the drain electrode 16a is connected to the data bus lines 15. A second insulating film 17 is formed on the data bus lines 15 and the TFT 16, and the picture element electrode 18 is formed on the second insulating film 17.
The picture element electrode 18 is formed of a transparent conductive material such as indium-tin oxide (ITO), and electrically connected to a source electrode 16b of the TFT 16 through a contact hole 17a formed on the second insulating film 17. On the picture element electrode 18, a vertical alignment film 19 formed of polyimide or the like is formed.
On the other hand, on a glass substrate 20a (on the lower side in FIG. 2) which becomes a base for the opposing substrate 20, black matrices (light blocking films) 21, color filters 22, and a common electrode 23 are formed. The black matrices 21 are formed of metal such as chromium (Cr), or black resin, and placed at positions facing the gate bus lines 11, the data bus lines 15, and the TFTs 16 on the side of the TFT substrate 10. For the color filter 22, there are color filters of three different colors, i.e. red (R), green (G), and blue (B), and a color filter of any one color among red, green, and blue is placed in each picture element. The common electrode 23 is formed of a transparent conductive material such as ITO, and formed on (under in FIG. 2) the color filter 22. A surface of the common electrode 23 is covered with a vertical alignment film 24 formed of polyimide or the like.
To maintain a uniform cell gap between the TFT substrate 10 and the opposing substrate 20 (the interval therebetween), for example, bead-like spacers (not shown) with a uniform diameter are spread; and the TFT substrate 10 and the opposing substrate 20 are joined with sealant spread outside a display region (a region in which picture elements are arranged in matrix).
In the liquid crystal display device constituted in this manner, when a voltage is not applied between the picture element electrode 18 and the common electrode 23, liquid crystal molecules are aligned in the direction perpendicular to the substrate surfaces. In this case, light outputted from the backlight goes into the liquid crystal layer 30 through the first polarizing plate 31a, and blocked by the second polarizing plate 31b. In this case, display becomes a black display (dark display).
On the other hand, when a voltage is applied between the picture element electrode 18 and the common electrode 23, liquid crystal molecules are aligned in parallel to the substrate surfaces, and light outputted from the backlight comes to pass through the first polarizing plate 31a and the second polarizing plate 31b. That is, display becomes a white display (bright display). An applied voltage is controlled for each picture element, and thereby desired images can be displayed.
The inventors of the present application consider that the conventional display device described above has the following problems.
It is preferred that liquid crystals within a picture element region be operated by using only a voltage to be applied to a picture element electrode. However, in practice, an electric field occurs due to signals passing through the gate bus lines 11 and data bus lines 15; and liquid crystal molecules in the vicinities of the gate bus lines 11 and the data bus lines 15 are operated with this electric field. Hereinafter, an area, in which liquid crystal molecules are operated with signals passing through the gate bus lines 11 and the data bus lines 15, is referred to as an abnormal operation area.
Given that the widths of the black matrices 21 are the same as those of the gate bus lines 11 and the data bus lines 15, the display quality is significantly deteriorated due to phenomena in which light passes through an abnormal operation region irrespective of being in black display, a hold after-image occurs, and the like. Thus, in general, the widths of the black matrices 21 are set to the width that are larger than those of the gate bus lines 11 and the data bus lines 15 by the total of the width of an abnormal operation region, and a process margin. However, as shown in FIG. 3, when a large force is exerted on the liquid crystal panel 1 by pressing the same with a finger, or the like, the TFT substrate 10 and the opposing substrate 20 are displaced, and abnormal operation regions A are strayed from the black matrices 21, hence deteriorating the display quality.
To prevent the above problem, it may be considered that the TFT substrate and the opposing substrate are joined with thermosetting resins scattered within a display region. For example, by coating bead-like spacers with thermosetting resin in advance, the TFT substrate and the opposing substrate can be joined with the resin coated on the spacers when setting sealant with heat. In this case, a process of applying a thermosetting resin to the TFT substrate and the opposing substrate is eliminated, and, hence, the increase of the number of processes is avoided. However, to join the TFT substrate and the opposing substrate with a sufficient strength, a large quantity of thermosetting resin is required; light is blocked from passing through a portion to which a thermosetting resin is stuck; and, hence, a problem that an aperture ratio is reduced and the screen becomes dark arises.